1. Field of the Invention
The present invention relates to an optical pickup apparatus. More particularly, present invention relates to an optical pickup apparatus using a diffraction element having diffraction gratings formed on both sides thereof in different structures to read and write information from and to optical data media of different formats and a method thereof.
2. Description of the Related Art
The optical pickup apparatus has been used in compact disc players (CDPs), digital versatile disc or digital video disk players (DVDPs), CD-ROM drives, DVD-ROM drives to write and read information to and from an optical data media in a non-contact manner. As the optical pickup apparatus writes data to an optical disc, the optical pickup apparatus emits a laser beam onto the surface of the optical disc to form a pitch. As the optical pickup apparatus reads the written data, the optical pickup apparatus optically reads data from the pitch formed in the optical disc, and outputs an electrical signal.
The optical pickup apparatus in a DVD player that uses high-density DVDs has to be able to read and write information from and to compact discs (CDs) and CD-Recordables (CD-Rs), for compatibility, in addition to the DVDs.
However, the thickness of a DVD is standardized differently from that of a CD in structural aspects such as a disc tilt tolerance error, the number of openings for objective lenses, and so on. The CD has a thickness of 1.2 mm, whereas the DVD has a thickness of 0.6 mm. Further, in terms of standards for writing, a 780 nm wavelength beam is used for CDs, whereas a 650 nm wavelength beam is used for DVDs. Thus, general CD players cannot write information to a DVD due to such a difference, so it is necessary to develop an optical pickup apparatus for DVDs that can be also used for existing CDs.
FIG. 1 is a diagram illustrating a conventional optical pickup apparatus for optical data media of different formats. Referring to FIG. 1, the conventional optical pickup apparatus has an optical module 10 formed with a first light source 11 and a first monitor optical detector 13 incorporated, a second light source 20, a second monitor optical detector 21 for adjusting the amount of light from a beam emitted from the second light source when reading and writing information, a half wavelength plate 25 for converting a p-polarized beam emitted from the second light source into an s-polarized beam, a combination prism 30 as a polarization beam splitter for changing the traveling paths of beams respectively emitted from the first and second light sources, a collimating lens 40 for converting the beams respectively emitted from the first and second light sources into parallel lights, a reflective mirror 45 for reflecting incident beams, a polarization hologram device 50 for splitting the beam traveling onto a disc 100 into plural beams, an objective lens 60 for focusing the split incident beams onto the disc 100, and an optical detector 80 for receiving beams reflected from the disc 100 and passing through the combination prism 30. If any of the first and second light sources is activated, the first light source emitting a 780 nm wavelength beam is used for CDs, and the second light source emitting a 650 nm wavelength beam is used for DVDs.
The polarization hologram device 50 is formed to work with a p-polarized beam of 650 nm wavelength. Further, a quarter wavelength plate is formed on one side of the polarization hologram device 50 facing the disc 100 to convert the incident s-polarized beam into a circularly polarized beam and convert the reflected beam from the disc 100 into the p-polarized beam. Accordingly, only the 650 nm wavelength beam reflected from the disc 100 is split into 10 beams by the polarization hologram device 50.
The third optical detector 80 receives the 10 split beams, and obtains data information and error information from the disc 100. The optical detector 80 has a plurality of sensors for detecting the incident beam that has been reflected from a DVD, and one sensor for detecting the incident beam that has been reflected from a CD.
As described above, the conventional optical pickup apparatus has to use the polarization hologram device 50 as a beam splitter specifically designed to write information to optical data media of different formats. Such a hologram device is not structured thick due to limitation to its weight and size, so it has a thickness of 0.68 mm. The thin hologram device has a disadvantage in that it bends or twists at high temperatures, causing optical signal errors.
Further, the conventional optical pickup apparatus is designed to enhance signal-writing operations (writing and reading) at a high temperature since it uses the combination prism 30 along the optical axes of the beams of different wavelengths as well as uses the same collimating lens 40 to form a simplified optical structure, but it is difficult to manufacture the combination prism 30, and advanced technologies are needed to assemble the combination prism 30. Further, the use of the polarization hologram device 50 causes difficulties in reducing assembly tolerance between the objective lens 60 and the polarization hologram device 50, difficulties exist in pickup calibrations due to the splitting of a beam into 10 beams, and the use of two laser diodes causes an optical axis error between the two beams. As stated above, the conventional optical pickup apparatus has a disadvantage of increased defects due to its complicated structure and poor assembly due to having lots of parts.
Further, the conventional optical pickup apparatus operates at a relatively high temperature since the power required for a laser to write data to a DVD is higher than that for writing to CDs. The parts in the pickup head are attached by ultraviolet bonding. Therefore, the attached portions are twisted or expanded at a high temperature. Thus, such deformities cause a beam passing through or reflecting from the parts to deviate from the optical detector, which degrades the signal writing operations at a high temperature. In order to compensate for such a disadvantage, non-essential parts need to be eliminated, or the number of split beams reaching the optical detector should be minimized.